Recently, the consumption of natural gas, such as liquefied natural gas (LNG) or liquefied petroleum gas (LPG), has been rapidly increasing throughout the world. Liquefied gas is transported in a gaseous state through onshore or offshore gas pipelines, or transported to a remote consumption place while being stored in a liquefied state inside a liquefied gas carrier. Liquefied gas, such as LNG or LPG, is obtained by cooling natural gas or petroleum gas to a cryogenic temperature (in the case of LNG, about −163° C.). Since the volume of liquefied gas is considerably reduced as compared to a gaseous state, liquefied gas is very suitable for a long-distance marine transportation
A liquefied gas carrier is designed to load liquefied gas, sail across the sea, and unload the liquefied gas at an onshore consumption place. To this end, the liquefied gas carrier includes a storage tank (also called “cargo hold”) that can withstand a cryogenic temperature of liquefied gas.
Examples of a marine structure provided with a storage tank capable of storing cryogenic liquefied gas may include vessels, such as a liquefied gas carrier and an LNG Regasification Vessel (LNG RV), or structures, such as an LNG Floating Storage and Regasification Unit (LNG FSRU) and an LNG Floating, Production, Storage and Off-loading (LNG FPSO).
The LNG RV is a self-propelled, floatable liquefied gas carrier equipped with an LNG regasification facility, and the LNG FSRU is a marine structure that stores LNG unloaded from an LNG carrier on the sea far away from the land and, if necessary, supplies the LNG to an offshore consumption place by gasifying the LNG. The LNG FPSO is a marine structure that refines extracted LNG on the sea, stores the LNG in a storage tank after direct liquefaction, and, if necessary, transships the LNG to an LNG carrier. The term “marine structure” as used herein is a concept including vessels, such as a liquefied gas carrier and an LNG RV, and structures, such as an LNG FPSO and an LNG FSRU.
Since the liquefaction temperature of natural gas is a cryogenic temperature of −163° C. at ambient pressure, LNG is likely to be vaporized even when the temperature of LNG is slightly higher than −163° C. at ambient pressure. In the case of a conventional LNG carrier, even though an LNG storage tank is thermally insulated, external heat is continuously transferred to LNG. Therefore, LNG is continuously vaporized and boil-off gas is generated within the LNG storage tank during the transportation of LNG by the LNG carrier.
The generated natural gas may increase the internal pressure of the storage tank and accelerate the flow of the natural gas due to the rocking of the vessel, causing structural problems. Therefore, it is necessary to suppress the generation of BOG.
Conventionally, in order to suppress the generation of BOG within the storage tank of the liquefied gas carrier, a method of discharging the BOG from the storage tank and burning the BOG, a method of discharging the BOG from the storage tank, reliquefying the BOG through a reliquefaction apparatus, and returning the BOG to the storage tank, a method of using the BOG as fuel for a vessel's propulsion engine, and a method of suppressing the generation of BOG by maintaining an internal pressure of a storage tank at a high level have been used solely or in combination.
In the case of a conventional marine structure equipped with a BOG reliquefaction apparatus, BOG inside a storage tank is discharged from the storage tank and then reliquefied through a reliquefaction apparatus in order to maintain a pressure of the storage tank at an appropriate level. In this case, before a reliquefaction process, the BOG is compressed to a low pressure of about 4 to 8 bara and then supplied to the reliquefaction apparatus. The compressed BOG is reliquefied through heat exchange with nitrogen cooled to a cryogenic temperature in the reliquefaction apparatus including a nitrogen refrigeration cycle, and the liquefied BOG is returned to the storage tank.
BOG may be compressed to a high pressure in order to increase the BOG reliquefaction efficiency. However, the LNG stored in the storage tank is maintained at an ambient pressure state, and therefore, if a pressure of the liquefied BOG is excessively high, flash gas may be generated when the BOG is returned to the storage tank. Consequently, the BOG needs to be compressed to the above-mentioned low pressure of about 4 to 8 bara, in spite of low reliquefaction efficiency.
Conventionally, as illustrated in FIG. 1, BOG generated in a storage tank, that is, NBOG, is supplied to a BOG compressor and is compressed to a low pressure of about 4 to 8 bara. Then, the low-pressure BOG is supplied to a reliquefaction apparatus using nitrogen gas as a refrigerant (the detailed description of Korean Patent Application Publication No. 10-2006-0123675 discloses that the BOG is compressed at about 6.8 bara, and the detailed description of Korean Patent Application Publication No. 10-2001-0089142 (relevant U.S. Pat. No. 6,530,241) discloses that the BOG is compressed at about 4.5 bara). Flash gas may be generated while the BOG liquefied in the reliquefaction apparatus, that is, LBOG, is returned to the storage tank. Hence, the BOG compressor necessarily compresses the BOG at a low pressure.
As a result, according to a typical BOG processing method, BOG generated in a storage tank is reliquefied through a reliquefaction apparatus and then returned to the storage tank. Till now, a basic concept for suppressing the generation of flash gas after the reliquefaction of BOG as much as possible is not to increase a pressure of BOG to be reliquefied.
A BOG reliquefaction apparatus uses a nitrogen refrigeration cycle disclosed in International Patent Publication Nos. WO 2007/117148 and WO 2009/136793 and Korean Patent Application Publication Nos. 10-2006-0123675 and 10-2001-0089142, and also uses other mixed refrigerant cycles. As described above, it is general that the conventional BOG reliquefaction apparatus reliquefies BOG by compressing the BOG to a pressure of about 4 to 8 bara. Also, it is well known in the art that it is technically inappropriate to compress BOG to a pressure higher than the above-mentioned pressure. This is because if BOG is reliquefied at a high pressure, the pressure of the BOG is lowered to about ambient pressure when the BOG is returned later to the tank, and thus, a large amount of flash gas (BOG) is generated.
Meanwhile, since the nitrogen refrigeration cycle uses nitrogen gas (N2) as a refrigerant, the liquefaction efficiency is low. Also, the mixed refrigerant cycle uses a refrigerant mixed with nitrogen and hydrocarbon gases as a refrigerant, the stability is low.
More specifically, a conventional offshore LNG reliquefaction apparatus for a vessel or an offshore plant reliquefies BOG by implementing a turbo-expander-type nitrogen reverse Brayton cycle. A conventional onshore LNG liquefaction plant liquefies natural gas by implementing a Joule-Thomson refrigeration cycle using a mixed refrigerant. The nitrogen reverse Brayton cycle used for the offshore LNG liquefaction apparatus is relatively simple in the configuration of the apparatus and thus is advantageous to a limited vessel or offshore plant, but has low efficiency. The mixed-refrigerant Joule-Thomson refrigeration cycle used for the onshore LNG liquefaction plant has relatively high efficiency but is complicated in the configuration of the apparatus because a separator needs to be used for separating a mixed refrigerant when a gaseous state and a liquid state coexist due to the feature of the mixed refrigerant. However, such a reliquefaction method has still been widely used.
Moreover, in the case of a marine structure equipped with a storage tank configured to store liquefied gas such as LNG, there is a need for extensive research and development of methods for efficiently processing BOG continuously generated in a storage tank and suppressing the generation of flash gas.